Our environment is contaminated by a vast array of potentially pathogenic microorganisms. Skin keratinocytes, epithelial cells lining the gastrointestinal tract, respiratory tract, genito-urinary tract all provide a physical barrier that protect against microbial intrusion into the body.
In addition, these epithelia contribute to the host defences by producing and secreting antimicrobials to limit access of bacteria and other microorganisms. These antimicrobial molecules constitute key components of the basic defence line of the innate immunity.
Defensins are one of the most important classes of antimicrobial peptides in humans. Defensins are produced by epithelial cells of the lung, skin, oral cavity, genitourinary, respiratory and gastrointestinal tract. Among these, there is the family of β-defensins including the defensin 1 (hBD1) and 2 (hBD2).
HBD1 is expressed in various mucosal surfaces such as oral mucosa, salivary gland, stomach, small intestine, colon, liver and pancreas. HBD2 is also present in epithelial cells at multiple mucosal surfaces including that of gastrointestinal tract. Moreover, these two defensins are also present in saliva and airway surface fluid (Cunliffe, R. N. and Mahida, Y. R. 2004, J Leukoc. Biol. 75:49-58).
HBD2 is present at very low levels in normal tissues, and its expression is up-regulated by bacteria and pro-inflammatory cytokines. Contrary to hBD2, HBD1 is constitutively expressed. HBD1 has never been shown to be consistently up-regulated by bacteria or inflammation (Ou, G., et al., 2009, Scand. J Immunol 69:150-161).
Probiotics are well known to be able to reinforce the various lines of gut defence: immune exclusion, immune elimination, and immune regulation. Probiotics are also known stimulate non-specific host resistance to microbial pathogens and thereby aid in their eradication.
However, despite this, the expression of the constitutive hBD1 has been reported as unaffected by probiotic bacteria (O'Neil, D. A. et al., J Immunol 163:6718-6724) and as very mildly upregulated by commensal (Escherichia coli) and pathogenic (Salmonella typhimurium) strains (Ou, G., et al., 2009, Scand. J Immunol 69:150-161).